"Energetic" compounds are used extensively in a wide variety of applications, e.g., in rocket propellants, explosive formulations, and the like. It is generally preferred that such materials have a high energy content yet be relatively insensitive to impact, such that accidents are avoided and energy is released only when intended. The requirements of insensitivity and high energy are in conflict, making the development of new energetic materials a difficult and challenging synthetic problem.
In developing new energetic compounds, a number of factors come into play. For example, heats of formation, density, melting and decomposition temperatures, carbon content and, generally, nitrogen content, are properties which must be considered. Energetic compounds should display good thermal and shock properties, have high heats of formation, and be straightforward to synthesize in bulk. It is generally preferred that an energetic compound have a melting point above about 100.degree. C., an exothermic heat of combustion, and a high decomposition temperature, with a relatively large separation between melting point and decomposition temperature preferred such that an energetic composition may be melt cast from the selected compound.
A number of energetic compounds are known as useful in the manufacture of oxidizers, explosives and the like. Energetic compounds have also been disclosed as useful to inflate automobile and aircraft occupant restraint bags. However, previously known materials are generally limited in one or more ways, e.g., they are overly impact-sensitive, difficult to synthesize on a large scale, not sufficiently energetic, or the like. In addition, energetic compositions used to inflate occupant restraint bags in automobiles and aircraft typically contain potentially toxic heavy metal igniter materials, e.g., mercury compounds, Pb(N.sub.3).sub.2 or the like.
The present invention provides a new class of compounds which overcomes the aforementioned limitations in the art. The energetic compounds to which the invention pertains are commonly referred to as "igniter materials" or "igniter compounds," i.e., compounds which act as primary explosives in an energetic composition. Igniter compounds should be thermally stable, typically up to a temperature of at least about 150.degree. C.; ideally, the compounds should also have a relatively high heat of formation, and be safe, economical and straightforward to synthesize in relatively high yield. The compounds now provided herein meet all of these criteria, and outperform conventional igniters such as lead azide and lead styphnate. In addition, it is important to note that the present compounds are thermally stable as well as shock-sensitive; normally, such compounds are either thermally stable or shock-sensitive, but not both.
The following references relate to one or more aspects of the present invention, and as such may provide background information not explicitly included herein. For example, U.S. Pat. No. 4,623,409 to Lee describes 1,1'-dinitro-3,3'-azo-1,2,4-triazole, a compound having the structure ##STR1## and stated to be useful as an explosive agent and as a candidate for use in high-energy propellant applications. The compound is synthesized by oxidizing 3-amino-1,2,4-triazole with potassium permanganate, followed by nitration of the intermediate so formed. U.S. Pat. No. 5,472,647 to Blau et al. describes substituted tetrazoles as gas generants. Examples of the tetrazoles are bicyclic compounds having the structure ##STR2## in which X, R.sub.1 and R.sub.2 represent H, methyl, ethyl, cyano, nitro, amino, tetrazolyl, a metal, or a nonmetallic cation of a nitrogen-containing base. Other references of interest include U.S. Pat. No. 5,501,823 to Lund et al. and U.S. Pat. No. 5,661,261 to Ramaswamy et al., which pertain to gas generating compositions, and U.S. Pat. Nos. 5,035,757 to Poole and 5,531,941 to Poole et al., which describe compositions for inflating automobile airbags. The latter compositions may contain nitrated tetrazoles; Poole et al. '775 mentions 5-nitrotetrazole and 5-nitroaminotetrazole, while Poole '757 mentions 3-nitro-1,2,4-triazole-5-one. Both patents disclose the use of metal salts such as strontium nitrate.